In recent years, demand for (zinc) plated steel strips, which enhance corrosion resistance, etc., have desirable aesthetic qualities, and in particular, are used as steel sheets for electronic products or automobiles, has rapidly increased.
FIG. 1 illustrates a process for plating a steel strip, particularly, a zinc-plating process.
For example, as shown in FIG. 1, a zinc plating process for steel strips is performed by allowing molten zinc to be attached to surfaces of a steel strip (for example, a cold-rolled steel strip) 100 while the steel strip passes through a snout and a zinc plating bath 110 after the steel strip is unwound from a pay-off reel and is thermally treated with a welding machine and a looper.
Here, a gas wiping device (for example, an air knife) 120 provided directly above the plating bath may spray a gas (for example, an inert gas or air) onto a surface of the steel strip to properly reduce the amount of zinc plated on the steel strip, thereby controlling the plating thickness of the steel strip.
Also, the plated steel strip may continuously pass through a sink roll 112 that allows the steel strip to pass through the plating bath 110 and adjusts a tension of the steel strip, a stabilizing roll 114, which are provided in the plating bath 110, and an upper transfer roll 130.
As shown in FIG. 1, the molten zinc filled in the zinc plating bath 110 may have a temperature of about 450° C. to about 460° C. The steel strip 100 passing through the plating bath 110 may have various types, widths, and thicknesses.
However, loads applied to (a roll shaft of) the sink roll 112 may be generally different according to types of steel strips. For example, a maximum load of about 500 kgf may be applied to both ends of the sink roll 112. Thus, when dynamical properties such as vibration occur, a maximum load of about 100 kgf may be applied to both ends of the sink roll 112 in a rotation direction of the sink roll 112.
Thus, while the plated steel strip 100 passing through the sink roll 112 and the stabilizing roll 114 passes through the upper transfer roll 130, vibrations in the steel strip 100 may occur even if the vibrations are varied according to the types, widths, or thickness of steel strips. Here, the occurrence of the vibration in the steel strip may cause a plating deviation between the gas wiping device 120 and the steel strip, resulting in a plating failure.
On the other hand, when a curvature phenomenon (for example, a C-shaped curvature or S-shaped curvature phenomenon in which a central portion of the steel strip is recessed or curved in a width direction of the steel strip) in which the steel strip is non-uniform in shape occurs, a plating deviation in the width direction of the steel strip may occur, thus resulting in the plating failure.
Thus, as shown in FIG. 1, at least one steel strip stabilizing apparatus (a so-called a “steel strip damping apparatus”) 140 for correcting the shape of the steel strip or suppressing vibration in the steel strip may be disposed between the gas wiping device 120 and the upper transfer roll 130.
The steel strip stabilizing apparatus 140 may damp (suppress) the vibrations in the plated steel strip or control the curvature shape in the steel strip to transfer the steel strip in a state in which the steel strip is flat, thereby preventing the plating deviation from occurring.
Although schematically shown in FIG. 1, the steel strip stabilizing apparatus 140 according to the related art may damp vibrations in the steel strip or correct the shape of the steel strip by using a mechanical touch roll that is in contact with the steel strip or spraying a gas onto the steel strip.
However, in the case of using the mechanical touch roll, since the roll contacts the surface of the transferred plated steel strip in a state in which the molten zinc is not completely attached (dried) to the surface of the steel strip by passing through the gas wiping device, a surface roll marker may be easily formed on the surface of the plated steel strip, and particularly, foreign matters may be attached to the surface of the steel strip by using the touch roll as a medium to cause quality defects in the plated steel strip.
For example, since most steel strips for vehicles are used in vehicle frames, the surface defects in the steel strip may cause significant quality defects in products. Also, the contact type roll may cause vibrations and noise due to abrasion thereof and also increase vibrations in the transferred plated steel strip due to unstable rotation thereof.
The related-art method for damping vibrations in the steel strip or correcting the shape of the steel strip by spraying the gas onto the steel strip may have limitations in which vibration suppression and shape correction in the steel strip are inefficient, and particularly, if the gas is sprayed onto the surface of the steel strip in the state where a plating solution is completely dried and thus is not attached to the surface of the steel strip, it may have an influence on the plating thickness of the steel strip.
Accordingly, a technique which enables the steel strip to be corrected in shape and damped (suppressed) in vibrations through a steel strip non-contact manner instead of the mechanical contact or gas spraying manner is required. For this, a method using electromagnetic force has been proposed as the other method in the related art.
However, in the case of the related-art method using the electromagnetic force, even in the case that the steel strip is suppressed in vibrations or corrected in shape through magnetic attraction force with respect to the steel strip in the non-contact manner, this may be merely a simple configuration in which a magnet block for generating magnetic fields (magnetic force) is disposed adjacent to the steel strip. Also, since the magnet block has a small unit area, the magnet block may cause stress concentration in the steel strip when the steel strip is damped in vibration and corrected in shape.
For example, in a case of a thin film having a thin thickness of about 0.6 t, a steel strip may be dented to cause surface defects of the steel strip.
Furthermore, as demand for plated steel strips used as steel strip for vehicles is rapidly increasing, large-scaled plating equipment and high-speed plating may be required. However, the related-art steel strip stabilizing apparatus using the simple magnet block structure may have limitations in use.